Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability and their ability to generate high torque at low speed. Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require reductant-rich conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions.
Diesel oxidation catalysts (DOCs) and catalyzed soot filter (CSF) catalysts are typically platinum-based or platinum-palladium-based and are used to accelerate NO oxidation, generating NO2 which in turn oxidizes the soot, which may be trapped in a downstream particulate filter or in the CSF. For a heavy-duty diesel (HDD) system involving a Selective Catalytic Reduction (SCR) catalyst downstream of the DOC or CSF, maintaining a relatively stable DOC-out or CSF-out NO2/NOx ratio leading into the SCR is highly desirable for accurate calibration of urea injection, which leads to improved NOx abatement at the tailpipe of the exhaust system.
Pt-based or Pt-dominated DOCs and CSF catalysts are known to display various degrees of degreening, that is, fresh state of the art catalysts experience enhanced activity only after being in use for a period of time, which means that during the degreening period, lower NO2 is generated from NO than the intrinsic activity of the catalysts would allow. Such a significant degreening effect is undesirable for maintaining the performance of the downstream SCR catalyst (as it results in an unstable NO2/NOx ratio leading into the SCR catalyst over time).
Degreening of DOCs and CSF catalysts under exhaust conditions can be simulated by treatment in an exhaust gas mixture at temperatures typically of about 450 to about 500° C., for a duration in the range of about 1 to about 4 hours. Such high temperature conditions are not always predictably achieved during regular lean engine operation, which means that the point at which the catalysts are fully degreened and produce a stable amount of NO2 varies from vehicle-to-vehicle. Under HDD operating conditions, for example, it is common that the engine-out exhaust temperature does not exceed 300° C. for a long period of time. After full degreening of a catalyst on a diesel vehicle, the catalyst performance is relatively stable until long term degradation slowly occurs over thousands of hours of use and/or by exposure to catalyst poisons, such as sulfur and phosphorous.
There is a need to develop high performance DOCs and CSF catalysts that are degreened during production rather than during use, thereby facilitating stable NOx conversion with downstream systems such as those comprising an SCR catalyst.